1. Field of the Invention
The present invention relates to piezoelectric film type actuators comprising a piezoelectric film and electrode films laminated integrally on the thin substrate, to be used in ink jet printer heads, microphones, vibrators, oscillators, various kinds of displacement sensors, pumps and switches.
2. Prior Art
In recent years, it has been required to control the positioning of elements on the sub-micron order in the precision and optical industries. To this end, piezoelectric film type actuators have been used, utilizing displacement caused by the inverse piezoelectric effect and electrostrictive effect or the reversed phenomenon generated when electric field is applied to piezoelectric or electrostrictive materials such as ferroelectrics.
In ink jet printer heads, for instance, piezoelectric film type actuators of unimorph and bimorph types have been used, but more compact actuators with higher density of piezoelectric film elements, with lower operable voltage and higher response speed, and with other higher performance have been needed.
The typical structure of a piezoelectric film type actuator is shown in FIG. 4. This conventional actuator is, generally called unimorph type, comprised of a thin substrate 21 made of insulating ceramics on which a lower electrode film 22, a piezoelectric film 23 and an upper electrode film 24 are laminated in this order into one piece, by using a filmmaking technique such as the screen printing method.
This piezoelectric/electrostrictive actuator, comprised of the thin substrate 21 and the piezoelectric/electrostrictive actuating part 25 formed on said thin substrate by laminating a lower electrode film 22, a piezoelectric film 24 and a upper electrode film 23, can obtain large displacement by traverse effect of the distortion induced by electric field, and is operable with low voltage and quick in response.
However, in these actuators, because wiring patterns 26 and 27 must be formed on the thin substrate 21 to maintain electric continuity to the lower electrode film 22 and the upper electrode film 24, if a plurality of piezoelectric/electrostrictive driving parts 25 are formed on the thin substrate 21, it becomes difficult and complex to draw wiring patterns 26 and 27 to the electrode films 22 and 24 on the substrate and the complexly patterned wires may affect the displacement of the thin substrate 21 adversely.
In order to solve this problem, it has been proposed to create a piezoelectric/electrostrictive actuator by forming a thin substrate with conductive ceramics to enable the thin substrate to serve as an electrode, on which a piezoelectric film and the other electrode film are directly laminated integrally. This piezoelectric/electrostrictive actuator may exhibit an advantage that wiring patterns are not needed to be printed on the substrate, which serves as one of the electrodes.
However, if a piezoelectric film is directly laminated onto a thin substrate, there is another problem that the required piezoelectric/electrostrictive property can not be obtained because the piezoelectric film 23 is not densified enough as it is heat treated for lamination.
To describe specifically, when heat treated, the previously sintered substrate tends to expand, and then the piezoelectric film formed on the substrate shrinks during sintering. Thus, the expansion of the substrate prevents contracting piezoelectric film, resulting in hindering the piezoelectric film to be made dense. Accordingly, in this method, the required piezoelectric/electrostrictive characteristics can not be obtained.
There is also another problem in that the piezoelectric film is heated. Namely, the conductive ceramic constituting the thin substrate reacts with the piezoelectric film, then deteriorating its piezoelectric/electrostrictive property.
Moreover, when conductive ceramics constituting the thin substrate is alumina or zirconia ceramics, which contains a conductive additive, if the amount of the conductive additive is excessive, the thin substrate becomes significantly weak, and not strong enough to be used as the vibrating plate. Thus, the specific electric resistance cannot be lowered sufficiently. Accordingly, it is difficult to use such ceramics as an electrode material.